Large-scale coherent orientations of quasar polarisation vectors: interpretation in terms of axion-like particles

The observation of redshift‐dependent coherent orientations of quasar polarisation vectors over cosmological distances in some regions of the sky is reviewed. Based on a good‐quality sample of 355 measured quasars, this observation seems to infer the existence of a new effect acting on light propagation on such huge distances.A solution in terms of nearly massless axion‐like particles has been proposed in the literature and its current status is discussed.